Bonjour,
2x + 4x + 12x = 144
18x = 144
x = 144/18 = 8
6x + 4x + 40 = 520
10x + 40 = 520
10x = 520 - 40
10x = 480
x = 480/10 = 48
8x + 10 = 10x + 4
8x - 10x = 4 - 10
-2x = -6
x = -6/-2 = 6/2 = 3
10x - 14 = 16x - 26
10x - 16x = - 26 + 14
-6x = -12
x = -12/-6 = 12/6 = 2